You aren't missing anything that isn't available now somewhere else. We visited the original Discovery Island once or twice. It was okay. It was too much of a trug for most people to do so and I cannot imagine there were enough visitors to justify its existence. I originally thought it was strange that they moved the name of the island to Animal Kingdom Park, but it actually makes sense when you realize just how much of Discovery Island at Animal Kingdom Park is devoted to animal encounters, including otters, kangaroos, tortoises, macaws, etc.